US President Barack Obama has signed into law a $633 billion dollar defense bill despite warnings from rights campaigners that the measure will tie his hands on closing Guantanamo Bay.

Obama signed the bill, which among many measures, authorizes funding for the war in Afghanistan and boosts security at diplomatic missions worldwide following the attack on the US consulate in Benghazi in September.

It also tightens sanctions against Iran, increases funding for special forces and requires a report from the administration on options for stopping Syrian President Bashar al-Assad using air power against civilians.

But the bill also contains measures designed by Congress to thwart his promise to close the war on terror camp in Cuba, and which initially prompted Obama to warn he could wield his veto to reject the whole law.

Obama said he signed the measure because it authorized "essential support" for serving members of the armed forces and renewed vital programs necessary to maintaining the strongest military in the world.

However, he criticized clauses in the legislation that would renew for one year the ban on using taxpayer funds to transfer Guantanamo Bay inmates to the United States and makes it more difficult to repatriate them abroad.

In a separate signing statement added to the bill, the president accused Congress of substituting political determinations on Guantanamo Bay suspects for evaluations by law enforcement and anti-terror professionals.

He warned that his administration would interpret the law in accordance with a judgment about whether the intervention of lawmakers conflicted with his own constitutional powers to conduct foreign and national security policy.

Obama only has the power to veto an entire bill, not to take out clauses with which he disagrees piecemeal, and he was unwilling to unleash the chaos for the defense establishment that rejecting the entire law would have caused.

Human rights groups however were disappointed at the president's move.

"It's not encouraging that the President continues to be willing to tie his own hands when it comes to closing Guantanamo," said Dixon Osburn of Human Rights First.

"The injustice of Guantanamo continues to serve as a stain on American global leadership on human rights."

Frank Jannuzi, Deputy Executive Director of Amnesty International USA warned that "solutions for ending human rights violations, not excuses, must be found."

"This law makes it harder for the President to fulfill his promise to close the Guantanamo detention facility, perpetuating a grave injustice against the detainees held without charge or fair trial," he said.

The bill authorizes $527.4 billion for the base Pentagon budget; $88.5 billion for overseas contingency operations including the war in Afghanistan; and $17.8 billion for national security programs in the Energy Department and Defense Nuclear Facilities Safety Board.